Add Telegram backend documentation (#357)

* Initial plan

* Add Telegram backend documentation

Co-authored-by: nijel <[email protected]>

* Update docs/backends/telegram.rst

Co-authored-by: Copilot <[email protected]>

* Update docs/backends/telegram.rst

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: nijel <[email protected]>
Co-authored-by: Michal Čihař <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: 3 people

docs/backends/index.rst

@@ -169,6 +169,7 @@ Social backends
    strava
    stripe
    taobao
+   telegram
    thisismyjam
    trello
    tripit

docs/backends/telegram.rst

@@ -0,0 +1,46 @@
+Telegram
+========
+
+Telegram uses a widget-based authentication method for login.
+
+- Create a bot using `BotFather`_ on Telegram to get a bot token.
+
+- Add the Telegram backend to ``AUTHENTICATION_BACKENDS``::
+
+    AUTHENTICATION_BACKENDS = (
+        ...
+        'social_core.backends.telegram.TelegramAuth',
+        ...
+    )
+
+- Fill the ``Bot Token`` value in the settings::
+
+    SOCIAL_AUTH_TELEGRAM_BOT_TOKEN = ''
+
+- Add the Telegram Login Widget to your login page. The widget should be configured
+  to send authentication data to your callback URL, which should be something like
+  ``http://example.com/complete/telegram/`` replacing ``example.com`` with your domain.
+
+- The Telegram Login Widget can be added using the following HTML::
+
+    <script async src="https://telegram.org/js/telegram-widget.js?22"
+            data-telegram-login="YOUR_BOT_USERNAME"
+            data-size="large"
+            data-auth-url="http://example.com/complete/telegram/"
+            data-request-access="write"></script>
+
+  Replace ``YOUR_BOT_USERNAME`` with your bot's username (without the @ symbol)
+  and update the ``data-auth-url`` to match your domain.
+
+- The authentication process verifies the data integrity using HMAC-SHA256 with
+  the bot token. Authentication data is considered valid for 24 hours from the
+  ``auth_date`` timestamp.
+
+- The backend extracts the following user information:
+  - User ID (required)
+  - Username
+  - First name
+  - Last name
+  - Photo URL (if available)
+
+.. _BotFather: https://t.me/botfather